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(nasm.info)Section B.4.114

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B.4.114. `FYL2X', `FYL2XP1': Compute Y times Log2(X) or Log2(X+1)
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     FYL2X                         ; D9 F1                [8086,FPU]
     FYL2XP1                       ; D9 F9                [8086,FPU]

   `FYL2X' multiplies `ST1' by the base-2 logarithm of `ST0', stores
the result in `ST1', and pops the register stack (so that the result
ends up in `ST0'). `ST0' must be non-zero and positive.

   `FYL2XP1' works the same way, but replacing the base-2 log of `ST0'
with that of `ST0' plus one. This time, `ST0' must have magnitude no
greater than 1 minus half the square root of two.